Zero-voltage-switched (ZVS), full-bridge (FB), phase-shifted (PS) converters are commonly used for DC-DC conversion because of the several advantages that they offer over other approaches. These advantages include high efficiency, i.e., reduced duty cycle loss, due to zero-voltage-switching, or “soft” switching, a relatively small circulating energy, and constant frequency operation allowing for simple control of the converter. DC-DC converters of this type are typically incorporated in a wide range of DC power supplies having various applications such as in battery chargers.
One disadvantage of the conventional ZVS-FB-PS converter is the dependence of the ZVS condition on the output load. At high output loads, a resonant inductor is typically used for storing energy and charging the stray and internal capacitance of the converter's switches. For light or zero loads, the ZVS condition is lost as is the high efficiency of this type of converter. To increase the efficiency for light or zero loads, a large commutating inductor is provided in series with the converter's power transformer to permit ZVS operation over a wide range of loads. However, the incorporation of this large inductor gives rise to very high conduction loses when the load is high and also results in a decrease in the effective duty-cycle because of slower changes in the primary current polarity. This increased inductance also gives rise to severe voltage ringing across the secondary-side output rectifiers due to resonance between the inductance and the junction capacitance of the rectifier.
Various other approaches have been proposed for increasing the efficiency of the ZVS-FB-PS DC-DC converter under the full range of output load conditions. For example, “A New Full Bridge Zero Voltage Switched Phase Shifted DC-DC Converter with Enlarged Duty Cycle and ZVS Range”, by J. Beirante and B. Borges, published in 3rd. Conference on Telecommunications CONFTELE 2001, Apr. 23-24, 2001, Figueira da Foz, Portugal, discloses a converter having an LCC auxiliary circuit connected to one end of the primary winding of the power transformer between the middle point of a voltage capacitor divider and the middle point of the passive-active leg of the bridge circuit. This arrangement allegedly reinforces the primary current during the passive-to-active transition thus increasing the available energy to achieve ZVS. Another approach is described in “A New Family of Full-Bridge ZVS Converters” by Y. Jang and M. Jovanovic, published in IEEE Applied Power Electronics Conf, (APEC) Proc., Miami Beach, Fla., Feb. 9-13, 2003, pp. 622-628. In this approach, two magnetic components, i.e., a transformer and a coupled inductor or a single-winding inductor, are used to respectively provide isolated output(s) and to store energy for ZVS. The volts-second products change in opposite directions with a change in phase shift between the two bridge legs. Both of these approaches substantially increase the complexity of the FB-ZVS converter and require careful consideration of various other circuit parameters for successful implementation.
The present invention addresses the aforementioned limitations of the prior art by providing a ZVS-FB-PS converter particularly adapted for use in a soft switching mode DC power supply which allows for efficient operation of the converter over a full range of output loads by maintaining the stray and internal capacitance of the converter's switches fully charged under all operating conditions. At light or no-loads, the center tap of the primary winding of the converter's power transformer undergoes the full voltage swing of the input alternating current for storing sufficient energy in the combination of an inductor and a pair of capacitors connected to the converter's switches provide ZVS operation under these load conditions.